Various techniques have been developed for antennas and structures used in wireless communication devices. For example, PTL 1 discloses an antenna device whose resonant frequency is tunable with a high degree of precision. PTL 2 (which is equivalent to WO98/44590) discloses a feed network for antenna. PTL 3 discloses an electromagnetic wave propagation medium that has broadband phase response. PTL 4 discloses an antenna device using a microwave resonator device. PTL 5 (which is equivalent to WO2006/023195) discloses metamaterials, including lenses having negative refractive indices in a wide band, diffractive optical devices, and gradient index optical devices. PTL 6 discloses a microwave transmission line. NPL 1 and NPL 2 disclose split ring resonator antennas.
Metamaterials in which a conductor pattern having a certain structure is periodically arranged to artificially control propagation characteristics of electromagnetic waves propagating through the structure have been developed in recent years. Among known basic components of the metamaterials are resonators that use a C-shaped split ring which is a ring conductor one circumferential portion of which is cut. The split ring resonators can interact with magnetic fields to control an effective magnetic permeability.
On the other hand, there is demand for reduction of the whole size of electronic devices that have communication functionality (for example wireless communication devices) and accordingly antennas need to be reduced in size. Therefore, the use of split ring resonators to reduce the size of antennas has been proposed. For example, NPL 1 discloses a technique in which a split ring resonator is disposed near a monopole antenna to increase the effective magnetic permeability and reduce the size of the monopole antenna. NPL 2 discloses a technique in which split ring resonators are periodically disposed in a region between a patch and a ground plane of a patch antenna to increase the effective magnetic permeability and reduce the size of the patch antenna.
In relation to the techniques described above, PTL 1 discloses an antenna device in which a slot is formed in a conductor plate provided on a surface of a dielectric substrate and a stub is formed on the other surface of the dielectric substrate through a via in such a manner that the stub extends across the slot, thereby enabling precise tuning of resonant frequency.